Once an oil painting has dried its surface is a tough durable film, unlike the fragile surface of pastels. I recommend applying a coat of re-touch varnish after the surface has dried. After six months applying a final varnish will protect the painting from air-bourne contaminants.

The method I use for framing oil paintings done on paper is to mount the paper to a panel, oak plywood or masonite, using an archival spray adhesive or rubber cement, before inserting it into the frame.

I hope this helps.